Collisional Line Broadening and Collisional Depolarization of Spectral Lines: Similarities and Differences

Abstract

International audienceThe collisional width of a spectral line takes part in the frequency re- distribution of the scattered radiation in the line. Within the impact approximation, collisional line broadening parameters (widths and shifts), depolarization and polariza- tion transfer rates seem very similar: both include the effect of collisional transitions between the Zeeman sublevels of a given level, or between fine or hyperfine structure levels of a given term. However, there are important differences. On the one hand, for line broadening, the two levels connected by the radiative transition contribute to the broadening. There is also an interference term between the two levels of the line, which can be very important for collisions with neutral hydrogen. On the other hand, only one level or two close levels are concerned in the depolarization. Another difference lies in the fact that elastic cross-sections of the two levels contribute to the line broadening, whereas they do not contribute to the depolarization. The possibility to find some the- oretical relationships concerning depolarization versus collisional broadening will be shown to be impossible. The perturbation expansion of the collisional S matrix and the Van der Waals interaction potential are recalled to be unsuitable, since all the derived parameters are too small (by approximately a factor 2). Finally, in the light of a very recent paper, numerical relationships between line widths and level depolarization rates will be quoted